Key storage for sorted string tables using content addressable memory

ABSTRACT

A memory system includes a memory device comprising a content addressable memory (CAM) block comprising a plurality of key tables each storing a respective plurality of stored search keys. The memory system further includes a processing device that receives, from a requestor, an input search key and an indication of one of the plurality of key tables and identifies a match between the input search key and one of the plurality of stored search keys in the one of the plurality of key tables. The one of the plurality of stored search keys has an associated offset value indicating a location in a sorted string table (SSTable) corresponding to the one of the plurality of key tables. The processing device further reads the offset value from the one of the plurality of key tables and returns, to the requestor, the offset value read from the one of the plurality of key tables. The requestor can retrieve, from the location in the SSTable, data representing a value associated with the input search key.

TECHNICAL FIELD

Embodiments of the disclosure relate generally to memory sub-systems, and more specifically, relate to key storage for sorted string tables using content addressable memory (CAM) in a memory sub-system.

BACKGROUND

A memory sub-system can include one or more memory devices that store data. The memory devices can be, for example, non-volatile memory devices and volatile memory devices. In general, a host system can utilize a memory sub-system to store data at the memory devices and to retrieve data from the memory devices.

BRIEF DESCRIPTION OF THE DRAWINGS

The present disclosure will be understood more fully from the detailed description given below and from the accompanying drawings of various embodiments of the disclosure.

FIG. 1 illustrates an example computing system that includes a memory sub-system in accordance with some embodiments of the present disclosure.

FIG. 2 is a block diagram of a memory device in communication with a memory sub-system controller of a memory sub-system in accordance with some embodiments of the present disclosure.

FIG. 3 is a block diagram illustrating a key-value data storage system using content addressable memory (CAM) in a memory sub-system in accordance with some embodiments of the present disclosure.

FIG. 4 is a block diagram of a content addressable memory (CAM) block of a key-value data storage system implemented within a memory device in accordance with some embodiments of the present disclosure.

FIG. 5 is a block diagram illustrating a database 500 implemented using SSTables and associated key tables in content addressable memory (CAM) in accordance with some embodiments of the present disclosure.

FIG. 6 is a flow diagram of an example method of processing a search operation in a key-value data storage system implementing key storage for sorted string tables using content addressable memory (CAM) in a memory sub-system in accordance with some embodiments of the present disclosure.

FIG. 7 is a block diagram of an example computer system in which embodiments of the present disclosure can operate.

DETAILED DESCRIPTION

Aspects of the present disclosure are directed to key storage for sorted string tables using content addressable memory (CAM) in a memory sub-system. A memory sub-system can be a storage device, a memory module, or a hybrid of a storage device and memory module. Examples of storage devices and memory modules are described below in conjunction with FIG. 1. In general, a host system can utilize a memory sub-system that includes one or more components, such as memory devices that store data. The host system can provide data to be stored at the memory sub-system and can request data to be retrieved from the memory sub-system.

A key-value data storage system is designed for storing, retrieving, and managing a collection of records. Each record can include different fields, holding one or more values. The records are stored in a database and retrieved using a key that uniquely identifies the record, and can be used to find the record within the database. In contrast to relational databases, in a key-value data storage system, the data (i.e., the values) are stored as a single opaque collection, which can be formatted differently in every record. This flexibility of not being tied to a defined schema allows the key-value data storage systems to utilize far less memory than relational databases.

Certain key-value systems utilize an underlying unit of storage, referred to as a sorted string table (SSTable). An SSTable is a collection of key-value pairs that is typically is stored as a single file in a file system. The key-value pairs in the SSTable are sorted and indexed to allow efficient querying. Since an SSTable is immutable once created (i.e., cannot be modified, only deleted), many key-value systems organize a number of SSTables as a hierarchy of levels. Each level contains multiple SSTables, with newer key versions added to the SSTables in higher levels, causing the SSTables with older key versions to be compacted down to lower levels. The nature of such a hierarchy of levels, while effective for storing SSTables, can lead to relatively high search times. Especially as the number of SSTables and/or the number of levels in the hierarchy increases, identifying the SSTable containing a desired key-value pair can be time consuming, as the hierarchy of levels must be traversed in a specific order.

Aspects of the present disclosure address the above and other deficiencies by providing key storage for SSTables using content addressable memory (CAM) in a memory sub-system. CAM is a type of memory device that is often used in certain very high speed searching applications such as identifier (ID) and pattern matching. Generally, a CAM is searched by comparing input search data against a table of stored data entries and a memory address of matching data in the table is returned. In one embodiment, the memory sub-system can utilize a CAM architecture implemented in a NAND-type flash memory device to store a key table including a copy of the keys from a corresponding SSTable, which can be located on either the same or a different memory device, to provide both fast and high capacity search capability. Consistent with this architecture, the keys can be stored on strings of a NAND-type flash memory array. Contrary to NAND implementations, each bit of a data entry (i.e., a stored key) is mapped to a pair of memory cells that are configured to be complementary. That is, a first memory cell of the pair stores a bit value and a second memory cell of the pair stores an inverse of the bit value.

In one embodiment, a search pattern representing an input search word is input vertically on each word line corresponding to a string in the array of the CAM. In one embodiment, the input search word represents a search key utilized in the key-value data storage system. A single read operation compares the input search word (i.e., the search key) with all strings in the selected portion of the array, typically one sub-block in one or more planes, and identifies a storage address of matching data (i.e., a stored key). In one embodiment, each stored key in the CAM is stored along with an offset value indicating where the associated key-value pair is stored in the corresponding SSTable. Thus, in response to a query for a given search key applied to the CAM, an indication of a match can be returned along with the associated offset value. The requestor that originally provided the input search word (e.g., a host system coupled to the memory sub-system or a memory sub-system controller) can then issue a subsequent read request to the corresponding SSTable to read the key-value pair directly from the received offset value without the need to search the SSTable (e.g., using the index).

Advantages of this approach include, but are not limited to, improved performance in the memory sub-system. The NAND-based CAM architecture improves existing database search systems and search algorithms such as index memories in cloud networking and servers, by offloading the search key queries to the CAM, thereby reducing the processing load on the SSTable database. The total number of bytes transferred between the host system and the SSTable storage can be reduced since a data transfer from a CAM key table is approximately 8 bytes in size. Without using the CAM, an SSTable index is searched, with each read from the index typically including a 4 KB block. In addition, multiple queries to the NAND-based CAM key table can be issued in parallel to reduce the total search latency. Furthermore, the use of NAND-based CAM can lead to less DRAM or other volatile memory being utilized to implement the functionality of the key-value data storage system which can reduce cost and power utilization in the memory sub-system.

FIG. 1 illustrates an example computing system 100 that includes a memory sub-system 110 in accordance with some embodiments of the present disclosure. The memory sub-system 110 can include media, such as one or more volatile memory devices (e.g., memory device 140), one or more non-volatile memory devices (e.g., memory device 130), or a combination of such.

A memory sub-system 110 can be a storage device, a memory module, or a hybrid of a storage device and memory module. Examples of a storage device include a solid-state drive (SSD), a flash drive, a universal serial bus (USB) flash drive, an embedded Multi-Media Controller (eMMC) drive, a Universal Flash Storage (UFS) drive, a secure digital (SD) and a hard disk drive (HDD). Examples of memory modules include a dual in-line memory module (DIMM), a small outline DIMM (SO-DIMM), and various types of non-volatile dual in-line memory module (NVDIMM).

The computing system 100 can be a computing device such as a desktop computer, laptop computer, network server, mobile device, a vehicle (e.g., airplane, drone, train, automobile, or other conveyance), Internet of Things (IoT) enabled device, embedded computer (e.g., one included in a vehicle, industrial equipment, or a networked commercial device), or such computing device that includes memory and a processing device.

The computing system 100 can include a host system 120 that is coupled to one or more memory sub-systems 110. In some embodiments, the host system 120 is coupled to different types of memory sub-system 110. FIG. 1 illustrates one example of a host system 120 coupled to one memory sub-system 110. As used herein, “coupled to” or “coupled with” generally refers to a connection between components, which can be an indirect communicative connection or direct communicative connection (e.g., without intervening components), whether wired or wireless, including connections such as electrical, optical, magnetic, etc.

The host system 120 can include a processor chipset and a software stack executed by the processor chipset. The processor chipset can include one or more cores, one or more caches, a memory controller (e.g., NVDIMM controller), and a storage protocol controller (e.g., PCIe controller, SATA controller). The host system 120 uses the memory sub-system 110, for example, to write data to the memory sub-system 110 and read data from the memory sub-system 110.

The host system 120 can be coupled to the memory sub-system 110 via a physical host interface. Examples of a physical host interface include, but are not limited to, a serial advanced technology attachment (SATA) interface, a peripheral component interconnect express (PCIe) interface, universal serial bus (USB) interface, Fibre Channel, Serial Attached SCSI (SAS), a double data rate (DDR) memory bus, Small Computer System Interface (SCSI), a dual in-line memory module (DIMM) interface (e.g., DIMM socket interface that supports Double Data Rate (DDR)), etc. The physical host interface can be used to transmit data between the host system 120 and the memory sub-system 110. The host system 120 can further utilize an NVM Express (NVMe) interface, Open NAND Flash Interface (ONFI) interface, or some other interface to access components (e.g., memory devices 130) when the memory sub-system 110 is coupled with the host system 120 by the physical host interface (e.g., PCIe bus). The physical host interface can provide an interface for passing control, address, data, and other signals between the memory sub-system 110 and the host system 120. FIG. 1 illustrates a memory sub-system 110 as an example. In general, the host system 120 can access multiple memory sub-systems via a same communication connection, multiple separate communication connections, and/or a combination of communication connections.

The memory devices 130,140 can include any combination of the different types of non-volatile memory devices and/or volatile memory devices. The volatile memory devices (e.g., memory device 140) can be, but are not limited to, random access memory (RAM), such as dynamic random access memory (DRAM) and synchronous dynamic random access memory (SDRAM).

Some examples of non-volatile memory devices (e.g., memory device 130) include negative-and (NAND) type flash memory and write-in-place memory, such as a three-dimensional cross-point (“3D cross-point”) memory device, which is a cross-point array of non-volatile memory cells. A cross-point array of non-volatile memory can perform bit storage based on a change of bulk resistance, in conjunction with a stackable cross-gridded data access array. Additionally, in contrast to many flash-based memories, cross-point non-volatile memory can perform a write in-place operation, where a non-volatile memory cell can be programmed without the non-volatile memory cell being previously erased. NAND type flash memory includes, for example, two-dimensional NAND (2D NAND) and three-dimensional NAND (3D NAND).

Each of the memory devices 130 can include one or more arrays of memory cells. One type of memory cell, for example, single level cells (SLC) can store one bit per cell. Other types of memory cells, such as multi-level cells (MLCs), triple level cells (TLCs), quad-level cells (QLCs), and penta-level cells (PLCs) can store multiple bits per cell. In some embodiments, each of the memory devices 130 can include one or more arrays of memory cells such as SLCs, MLCs, TLCs, QLCs, or any combination of such. In some embodiments, a particular memory device can include an SLC portion, and an MLC portion, a TLC portion, a QLC portion, or a PLC portion of memory cells. The memory cells of the memory devices 130 can be grouped as pages that can refer to a logical unit of the memory device used to store data. With some types of memory (e.g., NAND), pages can be grouped to form blocks.

Although non-volatile memory components such as 3D cross-point array of non-volatile memory cells and NAND type flash memory (e.g., 2D NAND, 3D NAND) are described, the memory device 130 can be based on any other type of non-volatile memory, such as read-only memory (ROM), phase change memory (PCM), self-selecting memory, other chalcogenide based memories, ferroelectric transistor random-access memory (FeTRAM), ferroelectric random access memory (FeRAM), magneto random access memory (MRAM), Spin Transfer Torque (STT)-MRAM, conductive bridging RAM (CBRAM), resistive random access memory (RRAM), oxide based RRAM (OxRAM), negative-or (NOR) flash memory, and electrically erasable programmable read-only memory (EEPROM).

A memory sub-system controller 115 (or controller 115 for simplicity) can communicate with the memory devices 130 to perform operations such as reading data, writing data, or erasing data at the memory devices 130 and other such operations. The memory sub-system controller 115 can include hardware such as one or more integrated circuits and/or discrete components, a buffer memory, or a combination thereof. The hardware can include a digital circuitry with dedicated (i.e., hard-coded) logic to perform the operations described herein. The memory sub-system controller 115 can be a microcontroller, special purpose logic circuitry (e.g., a field programmable gate array (FPGA), an application specific integrated circuit (ASIC), etc.), or other suitable processor.

The memory sub-system controller 115 can be a processing device, which includes one or more processors (e.g., processor 117), configured to execute instructions stored in a local memory 119. In the illustrated example, the local memory 119 of the memory sub-system controller 115 includes an embedded memory configured to store instructions for performing various processes, operations, logic flows, and routines that control operation of the memory sub-system 110, including handling communications between the memory sub-system 110 and the host system 120.

In some embodiments, the local memory 119 can include memory registers storing memory pointers, fetched data, etc. The local memory 119 can also include read-only memory (ROM) for storing micro-code. While the example memory sub-system 110 in FIG. 1 has been illustrated as including the memory sub-system controller 115, in another embodiment of the present disclosure, a memory sub-system 110 does not include a memory sub-system controller 115, and can instead rely upon external control (e.g., provided by an external host, or by a processor or controller separate from the memory sub-system).

In general, the memory sub-system controller 115 can receive commands or operations from the host system 120 and can convert the commands or operations into instructions or appropriate commands to achieve the desired access to the memory devices 130. The memory sub-system controller 115 can be responsible for other operations such as wear leveling operations, garbage collection operations, error detection and error-correcting code (ECC) operations, encryption operations, caching operations, and address translations between a logical address (e.g., logical block address (LB A), namespace) and a physical address (e.g., physical block address) that are associated with the memory devices 130. The memory sub-system controller 115 can further include host interface circuitry to communicate with the host system 120 via the physical host interface. The host interface circuitry can convert the commands received from the host system into command instructions to access the memory devices 130 as well as convert responses associated with the memory devices 130 into information for the host system 120.

The memory sub-system 110 can also include additional circuitry or components that are not illustrated. In some embodiments, the memory sub-system 110 can include a cache or buffer (e.g., DRAM) and address circuitry (e.g., a row decoder and a column decoder) that can receive an address from the memory sub-system controller 115 and decode the address to access the memory devices 130.

In some embodiments, the memory devices 130 include local media controllers 135 that operate in conjunction with memory sub-system controller 115 to execute operations on one or more memory cells of the memory devices 130. An external controller (e.g., memory sub-system controller 115) can externally manage the memory device 130 (e.g., perform media management operations on the memory device 130). In some embodiments, memory sub-system 110 is a managed memory device, which includes a raw memory device 130 having control logic (e.g., local media controller 135) on the die and a controller (e.g., memory sub-system controller 115) for media management within the same memory device package. An example of a managed memory device is a managed NAND (MNAND) device.

In one embodiment, memory device 130 can be configured to include a content addressable memory (CAM) NAND key-value database 137. As described in more detail below, the CAM-NAND key-value database 137 can include a CAM block of one or more arrays of memory cells organized as strings. A string is a unit in a NAND-type flash memory device. NAND-type flash memory devices typically have 32 or more memory cells, where each memory cell is used to represent a bit value (e.g., 0 or 1). Thus, a string with 32 memory cells can represent 32 bits of data and a string with 64 memory cells can represent 64 bits of data. In a NAND-type flash memory block, individual strings are connected to allow storage and retrieval of data from selected cells. Typically, strings in the block are connected at one end to a common source line and at the other end to a bit line. Each string stores a data entry and comprises memory cells connected in series between a match line and a page buffer. That is, the CAM block includes multiple match lines and each match line is connected to one of multiple strings in an array. The match lines of the CAM block correspond to bit lines of the NAND block on which the CAM block is implemented. Within a given string, memory cells can be organized as complementary memory cell pairs. Each bit value of the data entry stored by a string is mapped to one of the complementary memory cell pairs in the string.

The CAM block can be searched by providing a search pattern as input to search lines of the CAM block. The search lines of the CAM block correspond to word lines of the NAND block on which the CAM block is implemented. In one embodiment, the match lines of the CAM block are precharged to facilitate searching. That is, prior to input of the search, a voltage signal is applied to the match lines of the CAM block. During a search operation, if the input search word matches any data entry stored by the CAM block, one or more matched lines (e.g., match lines corresponding to strings storing the matching data entry) become conductive and discharge a signal in response to the search pattern input at the search lines. If the search word does not match any stored entry, all match lines are non-conductive. Each match line is further connected to a page buffer (e.g., comprises one or more latch circuits) that receives a discharge signal and stores data indicating that matched data is stored along the connected match line.

In one embodiment, memory sub-system 110 further includes a search component 113 that facilitates searching of the CAM-NAND key-value database 137. Consistent with some embodiments, the search component 113 is included in the memory sub-system controller 115, as shown. For example, the memory sub-system controller 115 can include the processor 117 (e.g., a processing device) configured to execute instructions stored in the local memory 119 for performing the operations of the search component 113 described herein. In some embodiments, memory device 130 includes at least a portion of the search component 113. In some embodiments, the search component 113 is part of the host system 120, an application, or an operating system.

The search component 113 generates a search pattern based on a received input search word and causes control logic (e.g., local media controller 135) to input the search pattern vertically along search lines of the CAM block in CAM-NAND key-value database 137. In one embodiment, the data entries in the CAM block are part of a key table storing copies of the keys, as well as associated offset values, from a corresponding SSTable. Depending on the embodiment, if there are multiple SSTables, there can be a separate key table corresponding to each respective SSTable. The SSTable(s) can be maintained on memory device 130, on another memory device in memory sub-system 110, or on another memory device separate from memory sub-system, but communicably coupled to host system 120. If a data entry matching the input search word is stored by the CAM block, the search pattern causes a match line storing the data entry (also referred to as a “matched line”) to become conductive and since the match lines are precharged, a matched line provides a signal to a connected page buffer that indicates that the search word is stored thereon. A location (e.g., a storage address) of any matching data entry may be identified based on the signal provided by the matched line as a result of the string being conductive. More specifically, a page buffer connected to any matched line stores data in response to detecting a discharge signal that indicates that the matched datum is stored along the matched line. A component of the search component 113 or control logic (e.g., a read-out circuit) may read data from the page buffer. Based on the data read from the page buffer, the search component 113 outputs an indication of whether the search word is stored by the CAM block and an indicator of the offset value stored with the matching key. In one embodiment, the offset value represents an address in the file where the key-value pair of the SSTable is stored. That offset value can be returned to a requestor (e.g., host system 120) that originally provided the input search word for use in a read operation performed on the SSTable. Further details with regards to the operations and structure of search component 113 and CAM-NAND key-value database 137 are described below.

FIG. 2 is a simplified block diagram of a first apparatus, in the form of a memory device 130, in communication with a second apparatus, in the form of a memory sub-system controller 115 of a memory sub-system (e.g., memory sub-system 110 of FIG. 1), according to an embodiment. Some examples of electronic systems include personal computers, personal digital assistants (PDAs), digital cameras, digital media players, digital recorders, games, appliances, vehicles, wireless devices, mobile telephones and the like. The memory sub-system controller 115 (e.g., a controller external to the memory device 130), may be a memory controller or other external host device.

Memory device 130 includes an array of memory cells 250 logically arranged in rows and columns. Memory cells of a logical row are typically connected to the same access line (e.g., a word line) while memory cells of a logical column are typically selectively connected to the same data line (e.g., a bit line). A single access line may be associated with more than one logical row of memory cells and a single data line may be associated with more than one logical column. Memory cells (not shown in FIG. 2) of at least a portion of array of memory cells 250 are capable of being programmed to one of at least two target data states. In one embodiment, the array of memory cells 250 includes CAM block 252 and SSTable 254. As described herein, the CAM block 252 includes a number of storage keys stored vertically in strings to which a received search key can be compared. When a match is determined, the matching storage key in CAM block 252 includes an offset value pointing to a location of corresponding key-value pair in SSTable 254 which can be retrieved and returned to the requestor. In one embodiment, memory device 130 organized into multiple planes. In one embodiment, for example, memory device 130 includes four planes. In other embodiments, there can be more or fewer planes, however. Each of the planes can be configured to include one or more CAM blocks 252 and SSTable 254. In one embodiment, there can be one key table in CAM block 252 corresponding to each SSTable 254. In another embodiment, CAM block 252 and SSTable 254 are not co-located on memory device 130. For example, SSTable 254 can be located on a separate memory device.

Row decode circuitry 208 and column decode circuitry 210 are provided to decode address signals. Address signals are received and decoded to access the array of memory cells 250. Memory device 130 also includes input/output (I/O) control circuitry 212 to manage input of commands, addresses and data to the memory device 130 as well as output of data and status information from the memory device 130. An address register 214 is in communication with I/O control circuitry 212 and row decode circuitry 208 and column decode circuitry 210 to latch the address signals prior to decoding. A command register 224 is in communication with I/O control circuitry 212 and local media controller 135 to latch incoming commands.

A controller (e.g., the local media controller 135 internal to the memory device 130) controls access to the array of memory cells 250 in response to the commands and generates status information for the external memory sub-system controller 115, i.e., the local media controller 135 is configured to perform access operations (e.g., read operations, programming operations and/or erase operations) on the array of memory cells 250. The local media controller 135 is in communication with row decode circuitry 208 and column decode circuitry 210 to control the row decode circuitry 208 and column decode circuitry 210 in response to the addresses.

The local media controller 135 is also in communication with a cache register 242. Cache register 242 latches data, either incoming or outgoing, as directed by the local media controller 135 to temporarily store data while the array of memory cells 250 is busy writing or reading, respectively, other data. During a program operation (e.g., write operation), data may be passed from the cache register 242 to the data register 244 for transfer to the array of memory cells 250; then new data may be latched in the cache register 242 from the I/O control circuitry 212. During a read operation, data may be passed from the cache register 242 to the I/O control circuitry 212 for output to the memory sub-system controller 115; then new data may be passed from the data register 244 to the cache register 242. The cache register 242 and/or the data register 244 may form (e.g., may form a portion of) a page buffer of the memory device 130. A page buffer may further include sensing devices (not shown in FIG. 2) to sense a data state of a memory cell of the array of memory cells 250, e.g., by sensing a state of a data line connected to that memory cell. A status register 222 may be in communication with I/O control circuitry 212 and the local memory controller 135 to latch the status information for output to the memory sub-system controller 115.

Memory device 130 receives control signals at the memory sub-system controller 115 from the local media controller 135 over a control link 232. For example, the control signals can include a chip enable signal CE #, a command latch enable signal CLE, an address latch enable signal ALE, a write enable signal WE #, a read enable signal RE #, and a write protect signal WP #. Additional or alternative control signals (not shown) may be further received over control link 232 depending upon the nature of the memory device 130. In one embodiment, memory device 130 receives command signals (which represent commands), address signals (which represent addresses), and data signals (which represent data) from the memory sub-system controller 115 over a multiplexed input/output (I/O) bus 234 and outputs data to the memory sub-system controller 115 over I/O bus 234.

For example, the commands may be received over input/output (I/O) pins [7:0] of I/O bus 234 at I/O control circuitry 212 and may then be written into command register 224. The addresses may be received over input/output (I/O) pins [7:0] of I/O bus 234 at I/O control circuitry 212 and may then be written into address register 214. The data may be received over input/output (I/O) pins [7:0] for an 8-bit device or input/output (I/O) pins [15:0] for a 16-bit device at I/O control circuitry 212 and then may be written into cache register 242. The data may be subsequently written into data register 244 for programming the array of memory cells 250.

In an embodiment, cache register 242 may be omitted, and the data may be written directly into data register 244. Data may also be output over input/output (I/O) pins [7:0] for an 8-bit device or input/output (I/O) pins [15:0] for a 16-bit device. Although reference may be made to I/O pins, they may include any conductive node providing for electrical connection to the memory device 130 by an external device (e.g., the memory sub-system controller 115), such as conductive pads or conductive bumps as are commonly used.

It will be appreciated by those skilled in the art that additional circuitry and signals can be provided, and that the memory device 130 of FIG. 2 has been simplified. It should be recognized that the functionality of the various block components described with reference to FIG. 2 may not necessarily be segregated to distinct components or component portions of an integrated circuit device. For example, a single component or component portion of an integrated circuit device could be adapted to perform the functionality of more than one block component of FIG. 2. Alternatively, one or more components or component portions of an integrated circuit device could be combined to perform the functionality of a single block component of FIG. 2. Additionally, while specific I/O pins are described in accordance with popular conventions for receipt and output of the various signals, it is noted that other combinations or numbers of I/O pins (or other I/O node structures) may be used in the various embodiments.

FIG. 3 is a block diagram illustrating a key-value data storage system using content addressable memory (CAM) in a memory sub-system, according to an embodiment. In one embodiment, the search component 113 receives an input search key 305 (e.g., a search word) and applies the input search key 305 to CAM block 252 to compare the input search key 305 to a number of stored search keys 352. In one embodiment, the search component 113, or other logic in memory sub-system 110, generates a search pattern based on the input search key 305. The input search key 305 can include a first sequence of bits (e.g., “1011”). The generated search pattern can include a first set of voltage signals representing the input search key 305 and a second set of voltage signals representing a second sequence of bits comprising an inverse of the first sequence of bits (e.g., “0100”). In one embodiment, the search component 113 or other logic includes an inverter to generate an inverse of the input search key and a level selector to generate the first and second signals. In generating the first and second voltage signals, the level selector can use a high voltage to represent a binary value of “1” and use a low voltage to represent a binary value of “0” where the high voltage is above a threshold voltage (Vt) and the low voltage is below Vt.

To search CAM block 252, the search component 113 inputs the search key 305 (i.e., the representative search pattern) vertically along search lines of the CAM block 252. Input of the search key 305 causes any complementary memory cell pairs representing a matching stored bit value to become conductive. If a string is storing matching data (i.e. a matching one of stored search keys 352), the entire string becomes conductive. Match lines in the CAM block 252 are precharged (e.g., connected to the high voltage), and because the match lines are precharged, input of the search key 305 on the search lines causes any match lines in the block that are storing matching data (e.g., one of stored search keys 352 that is identical to the search key 305) to output a discharge signal because the corresponding string is conductive. The discharge signal provides an indication that matching data (e.g., the input search key 305) is stored thereon. The discharge signal provides an indication that matching data is stored on the string connected to the match line.

Each string is connected between a match line and a page buffer (e.g., comprising one or more latch circuits) and the page buffer of a matched line stores data indicating matching data is stored along the matched line in response to the signal provided as a result of the match line discharging along the string. A page buffer can include one or more latch circuits. Physically, the page buffer(s) reside under or adjacent to the arrays of memory cells in which CAM block(s) 252 are implemented. A page buffer latches data based on the signal provided by a matched line when matching data is stored by the connected string that conducts the signal to the page buffer. The search component 113 reads data from the page buffer(s) and provides an indicator of whether the input search key 305 is stored in CAM block 252 being searched as output along with an offset value 312 stored with the matching stored key. In one embodiment, the offset value 312 represents a storage location in a file including SSTable 254, where a corresponding key-value pair is located.

In some embodiments, the search component 113 can sequentially search for matching data in the CAM block(s) 252 of multiple memory planes. In some embodiments, the search component 113 can search for matching data in the CAM block(s) 252 of the multiple memory planes in parallel. Parallel searching of the multiple memory planes allows all data entries stored among all CAM block(s) 252 of the planes to be searched in a single search operation rather than completing the search of all data entries in four separate search operations. Hence, parallel searching, as utilized in the embodiments described above, can allow the search component 113 to achieve an increase to search speed relative to embodiments in which sequential searching is utilized.

In some embodiments, data entries can be stored across two or more of the memory planes. In these instances, the search component 113 can simultaneously search for portions of matching data across two or more of the memory planes. Dividing data entries across planes allows for greater word size when compared to embodiments in which data entries are stored within a single plane. For example, if each of the CAM blocks 252 supports 64-bit words, dividing the data entries among all four planes would allow the memory device to support 256-bit words (4*64=256).

In one embodiment, the offset value 312 can be applied as an input to SSTable 254. The SSTable 254 can be implemented in NAND-type flash memory (e.g., on memory device 130), or other non-volatile memory, which is not arranged using a CAM architecture, and can include a number of stored key-value pairs 354. Depending on the embodiment, SSTable 254 can be co-located on the same die as CAM block 252, can be on a different memory device (i.e., memory die) in memory sub-system 110, or can be located separate from memory sub-system 110. In one embodiment, search component 113 or host system 110 can perform a read operation on SSTable 254 using the offset value 312 indicating a location where the data (i.e., an associated one of stored key-value pairs 354) corresponding to the input search key 305 is located. That data (i.e., value 330) can be returned to a requestor that originally provided the input search key 305 (e.g., a host system 120 coupled to the memory sub-system 110).

FIG. 4 is a block diagram of a content addressable memory (CAM) block of a key-value data storage system implemented within a memory device in accordance with some embodiments of the present disclosure. As shown, the CAM block 252 includes match lines 302-0 to 302-N, search lines 304-0 to 304-M, and inverse search lines 306-0 to 306-M. In this implementation, the match lines 302-0 to 302-N of the CAM block 252 correspond to bit lines of the NAND-type flash memory device and the search lines 304-0 to 304-M and inverse search lines 306-0 to 306-M of the CAM block 252 correspond to word lines of the NAND-type flash memory device.

Each of the match lines 302-0 to 302-N is connected to a string comprising a plurality of memory cells connected in series. For example, match line 302-0 is connected to a string comprising memory cells 308-0 to 308-X, where X=2M. Memory cells in each string of the CAM block 252 are configured to be complementary pairs. For example, with the string connected to match line 302-0, memory cells 308-0 to 308-X are programmed as complementary memory cell pairs 310-0 to 310-M.

Memory cell pairs are configured to be complementary in that one memory cell in the pair stores a data value (“0”) and the other memory cell in the pair stores an inverse of the data value (“1”). For example, memory cell pair 310-0 comprises two memory cells. A first memory cell stores a data bit value DATA, and a second memory cell 308-0 stores DATA, which is an inverse of the data bit value DATA. Also, as shown, search line 304-0 is connected to a control gate of the first memory cell and inverse search line 306-0 is connected to a control gate of the second memory cell.

Search line 304-0 receives a first signal SL representing a search bit value from an input search word and inverse search line 306-0 receives a second signal SL representing an inverse of the search bit value. If SL matches DATA and SL matches DATA, the memory cell pair 310-0 will be conductive from A to B. For example, TABLE 1 provided below is a truth table that defines the behavior of any given one of the memory cell pairs 310-0 to 310-M.

TABLE 1 SL SL DATA DATA CONDUCTIVE 0 1 0 1 YES 0 1 1 0 NO 1 0 0 1 NO 1 0 1 0 YES

In TABLE 1, “SL” is a search bit value, “SL” is an inverse of the search bit value, “DATA” is a stored bit value, and “DATA” is an inverse of the stored bit value. As shown, a complementary cell pair is conductive when the search data value matches the stored data value and the inverse of the search data value matches the inverse of the stored data value. In other instances, the memory cell pair 310 is non-conductive because the stored data does not match the search bit.

In one embodiment, each string in the CAM block 252 stores a data entry and each data bit value in a data entry is mapped to one of the memory cell pairs 310-0 to 310-M in the string. In this way, within each of the complementary memory cell pairs 310 in a string, a first memory cell stores a bit value from the data entry and a second memory cell stores an inverse of the bit value from the data entry.

In an example where the NAND-type flash memory device supports 128 bit strings (i.e., X is 128), the match line 302-0 is connected to memory cells 308-0 to 308-127, which stores 64 bit data entry comprising bit values D_(0,0)-D_(63,63). In this example, bit value D_(0,0) is mapped to memory cell pair 310-0 comprising memory cells 308-0 and 308-1. More specifically, memory cell 308-0 stores the bit value D_(0,0) and the complementary memory cell stores D _(0,0), which is the inverse of the bit value D_(0,0).

A search pattern 312 may be input vertically along search lines 304-0 to 304-M and inverse search lines 306-0 to 306-M. More specifically, search lines 304-0 to 304-M receive a first set of voltage signals SL_(0-M) representing a search word 305, and inverse search lines 306-0 to 306-M receive a second set of voltage signals SL _(0-M) representing an inverse of the search word. Input of the search pattern 312 along the search lines causes any string that stores matching data to be conductive because, as discussed above, each individual memory cell pair 310 in the string will be conductive. Because the match lines 302 are precharged, a conductive string allows the match line 302 to discharge. A page buffer connected to a conductive string latches data that indicates a location of matching data (i.e., the search word 206) in the CAM block 252.

The search component 113 outputs an indication of whether a search word 206 is stored by the CAM block 252 and an offset value (e.g., a memory address) of the matching data from a corresponding SSTable. In some embodiments, the search component 113 comprises a read-out circuit that reads data from the page buffers of the CAM block 252 to identify the location thereof.

In some embodiments, two page buffers in the CAM block 252 can be tied together to form a serial shift register. Consistent with these embodiments, the search component 113 shifts data out of a first page buffer to a second page buffer and the search component 113 comprises an output compare and counter component to track the number of shifts from one page buffer to the other to identify the location of matching data stored by the CAM block 252. In some embodiments, two page buffers may be tied together using a single transistor to form a shift register.

FIG. 5 is a block diagram illustrating a database 500 implemented using SSTables and associated key tables in content addressable memory (CAM) in accordance with some embodiments of the present disclosure. Database 500 is merely one example of the underlying datastore which can be utilized with the CAM key table approach described herein. The logical organization of the SSTables in database 500 should not be considered limiting, as there are a variety of local organizations with which these techniques can be used. Some examples of database 500 can include RocksDB, Apache Cassandra, ScyllaDB, LevelDB, and many others. In one embodiment, database 500 includes a number of SSTables organized in a hierarchy of levels (0-K). Each level contains multiple SSTables, with newer key versions added to the SSTables in higher levels, causing the SSTables with older key versions to be compacted down to lower levels. For example, Level 0 includes SSTables 502 and 504, which include the newest key versions in database 500. Each of SSTables 502 and 504 includes a range of keys (and associated values), which can be overlapping. For example, a given key pair can be present in each of SSTables 502 and 504. In other embodiments, Level 0 can have some other number of SSTables. Level 1 is the next level below Level 0, and includes SSTables 506, 508, 510, and potentially other SSTables. Each of SSTables 506, 508, and 510 include a range of keys, which are non-overlapping. Thus, a given key pair can be present in only one of SSTables 506, 508, or 510. SSTables 506, 508, and 510 can include the same or different keys as SSTables 502 and 504 in Level 0, but all keys in SSTables 506, 508, and 510 are older versions than in SSTables 502 and 504. Level 2 is the next level below Level 1, and includes SSTables 512, 514, 516, 518, and potentially other SSTables. Each of SSTables 512, 514, 516, and 518 include a range of keys, which are non-overlapping. There can be any number of levels present in database 500, up to Level K, which includes SSTables 520, 522, 524, 526, and potentially other SSTables. Each of SSTables 520, 522, 524, and 526 include a range of keys, which are non-overlapping. Database 500 can further include metadata (not shown) indicating the range of keys stored in each SSTable, an indication of a corresponding key table in CAM block 252, as well as any other metadata elements.

As described above, CAM block 252 can include a number of key tables 552-576, each corresponding to a respective SSTable in database 500. For example, key table 552 can correspond to SSTable 502, key table 554 can correspond to key table 504, and so on, up to key table 576 which corresponds to SSTable 526. In one embodiment, a copy of the keys from each respective SSTable can be stored in each corresponding key table in CAM block 252, along with an offset value indicating the location of the associated key-value pair in the corresponding SSTable. The values, from the key-value pairs, remain stored in the SSTables. Thus, a search of database 500, which would normally include traversing the hierarchy of levels in a specific sequential order (e.g., starting at Level 0, identifying the SSTable(s) storing the range of keys which would include the search key if present, searching that SSTable(s), moving down to Level 1, identifying the SSTable storing the range of keys which would include the search key if present, searching that SSTable, etc.) can be avoided. In one embodiment, search component can identify the SSTables in database which would include the search key if present (e.g., using the associated metadata), identify the corresponding key tables in CAM block 252, and search each of those corresponding key tables using parallel query operations. If a match is found between the input search key and a stored search key in one of those identified key tables, the associated offset value can be retrieved from the key table and returned to the requestor. The requestor can then perform a read operation to database 500 using the offset value to read the key-value pair directly from its storage location in the corresponding SSTable. In the case of a single key search (i.e., a “point query”), the requestor need only to read from the “newest” SSTable in database 500. When searching for a key prefix, however, the results from all the SSTables in database 500 that contain one or more entries matching the prefix are returned and merged, while discarding all but the newest of any duplicates entries.

In one embodiment, search component 113 includes a command library which can be utilized by a requestor, such as host system 110, to create, query, and delete the SSTables in CAM block 252. For example, the host system 110 can issue a command to create a key table corresponding to an existing or to be created SSTable. Search component 113 can create the key table, assign a unique key table identifier and return the key table identifier to host system 110. Host system 110 can further issue write commands to write keys, and associated offset values, from the SSTable to the key table in CAM block 252 (identified by the key table identifier). Once created, the host system 110 can issue query commands including an input search key (or key prefix) to the key table, with the understanding that a matching stored key, if available in database 500, would be found in the corresponding key table. In response, host system 110 can receive an indication of the match, if any, and an associated offset value indicating the location of the key-value pair in the SSTable. In addition, host system 110 can issue a delete command to delete a key table from CAM block 252 when no longer needed.

FIG. 6 is a flow diagram of an example method of processing a search operation in a key-value data storage system implementing key storage for sorted string tables using content addressable memory (CAM) in a memory sub-system in accordance with some embodiments of the present disclosure. The method 500 can be performed by processing logic that can include hardware (e.g., processing device, circuitry, dedicated logic, programmable logic, microcode, hardware of a device, integrated circuit, etc.), software (e.g., instructions run or executed on a processing device), or a combination thereof. In some embodiments, the method 500 is performed by search component 113 of FIG. 1. Although shown in a particular sequence or order, unless otherwise specified, the order of the processes can be modified. Thus, the illustrated embodiments should be understood only as examples, and the illustrated processes can be performed in a different order, and some processes can be performed in parallel. Additionally, one or more processes can be omitted in various embodiments. Thus, not all processes are required in every embodiment. Other process flows are possible.

At operation 605, the processing logic (e.g., search component 113) can receive the input search key 305 from a requestor (e.g., host system 120). In one embodiment, the input search key includes a first sequence of bits (e.g., “1001 1010 1011”). In one embodiment, the input search key is received as part of a request or query that further includes an indication of a key table, such as one of multiple key tables stored in a CAM block, such as CAM block 252 (e.g., formed from a NAND-type flash memory array of memory cells). In one embodiment, the request or query further includes an indication of two or more key tables that potentially include a match for the input search key. These two or more key tables can be associated with respective key ranges that would include a match for the input search key. In one embodiment, the request or query further includes an indication of a specific sub-block of one or more key tables that potentially include a match for the input search key. In one embodiment, the request or query further includes a key prefix (e.g., an indication of the first several bits of the input search key) rather than the entire search key. Such a request or query would instruct the processing logic to identify all of the keys from the key table that start with the key prefix.

At operation 610, the processing logic can generate a search pattern based on the first sequence of bits. In one embodiment, the search pattern comprises first set of voltage signals representing the input search key 305. That is, the first set of voltage signals represents the first sequence of bits. The search pattern further comprises a second set of voltage signals representing a second sequence of bits comprising an inverse of the first sequence of bits (e.g., “0110 0101 0100”). Accordingly, in generating the search pattern, the processing logic generates the second sequence of bits by inverting the input search key 305 and converts the first and second sequence of bits into the first and second sets of voltage signals, respectively. The processing logic may alternatively generate a first set of voltage signals based on the first sequence of bits and generate the second set of voltage signals by generating an inverse of the first set of voltage signals. In generating the first and second sets of voltage signals, the processing logic may use a high voltage to represent a binary value of “1” and use a low voltage to represent a binary value of “0” where the high voltage is above a threshold voltage (Vt) and the low voltage is below Vt.

At operation 615, the processing logic provides the search pattern to a CAM block. In one embodiment, the CAM block, such as CAM block 252, includes an array of memory cells organized into strings, with each string storing a respective stored search keys. A string includes a number of memory cells connected in series between a precharged match line and a page buffer. The match line is precharged in that it is connected to a voltage signal (e.g., representing a logical high state). The CAM block further includes a number of search lines, and each of the memory cells in a string are connected to one of the search lines.

As noted above, the memory cells in each string are organized as complementary memory cell pairs. Each bit value of a data entry stored by a string is mapped to a complementary memory cell pair in the string. In particular, a first memory cell stores the bit value and the second memory cell stores the inverse of the bit value. More specifically, a first memory cell stores a first charge representing the bit value and the second memory cell stores a second charge representing the inverse of the bit value.

In providing the search pattern to the search lines of the CAM block 252, the processing logic can provide a first signal representing a search bit value from the first bit sequence to a first search line connected to a first memory cell in a complementary memory cell pair and provide a second search signal representing an inverse of the search bit value to a second search line connected to a second memory cell in the complementary memory cell pair. If the input search key 305 is stored in the CAM block 252, input of the search pattern causes the string on which the input search word is stored to become conductive. Because matched lines are precharged, the conductive string allows the match line to discharge. That is, the string conducts a signal resulting from the match line discharging based on the input search key 305 matching the stored search key on the string connected to the match line. The conductive string provides the signal to a page buffer connected at the other end of the string. The page buffer latches data in response to the signal provided as a result of the match line discharging. The latched data indicates that the match line connected to the page buffer stores a data entry that is identical to the input search key 305.

At operation 620, the processing logic identifies a match between the input search key and one of the stored search keys in the identified key table. For example, the processing logic can identify one of the stored search keys 352 that matches the input search key 305. In one embodiment, the processing logic can determine whether any of stored search keys 352 match the input search key 305 by reading data from the page buffer of CAM block 252. The matching one of the plurality of stored search keys 352 has an associated offset value 312 in the CAM block 252. In one embodiment, where the request or query included an indication of two or more key tables, the processing logic can perform a search of those two or more key tables in parallel to identify the match between the input search key and the stored search key. In one embodiment, where the request or query included an indication of a specific sub-block of the key table, the processing logic can perform a search limited to that specific sub-block to identify the match between the input search key and the stored search key. In one embodiment, where the request or query included a key prefix, the processing logic can identify the first key from the key table that matches the key prefix (i.e., starts with the same bit string). Since the keys in the key table are sorted in numerical order (e.g., from smallest to largest), as they are also arranged in the corresponding SSTable, upon determining the offset value for the first key from the key table that matches the key prefix, the requestor can continue reading stored values from the SSTable, in order, until the stored keys stop matching the key prefix.

At operation 625, the processing logic reads the offset value 312 from the identified key table. In one embodiment, the processing logic can determine the offset value 312 based on the data read from the page buffer. The offset value 312 can include one or more memory addresses representing a location in an SSTable corresponding to the searched key table. The SSTable, for example, can be one of multiple SSTables in database 500, where each of the multiple SSTables has a corresponding key table in CAM block 252.

At operation 630, the processing logic returns the offset value read from the key table to the requestor. For example, the processing logic can provide the offset value 312 to a host system (e.g. host system 120). In one embodiment, the requestor can use the offset value 312 to retrieve, from the location in the corresponding SSTable, data representing one of stored values 354 (i.e., value 330) associated with the input search key 305. In another embodiment when a key prefix is utilized, the requestor reads stored values from the corresponding SSTable(s), in order, until the stored keys stop matching the key prefix.

FIG. 7 illustrates an example machine of a computer system 700 within which a set of instructions, for causing the machine to perform any one or more of the methodologies discussed herein, can be executed. In some embodiments, the computer system 700 can correspond to a host system (e.g., the host system 120 of FIG. 1) that includes, is coupled to, or utilizes a memory sub-system (e.g., the memory sub-system 110 of FIG. 1) or can be used to perform the operations of a controller (e.g., to execute an operating system to perform operations corresponding to search component 113 of FIG. 1). In alternative embodiments, the machine can be connected (e.g., networked) to other machines in a LAN, an intranet, an extranet, and/or the Internet. The machine can operate in the capacity of a server or a client machine in client-server network environment, as a peer machine in a peer-to-peer (or distributed) network environment, or as a server or a client machine in a cloud computing infrastructure or environment.

The machine can be a personal computer (PC), a tablet PC, a set-top box (STB), a Personal Digital Assistant (PDA), a cellular telephone, a web appliance, a server, a network router, a switch or bridge, or any machine capable of executing a set of instructions (sequential or otherwise) that specify actions to be taken by that machine. Further, while a single machine is illustrated, the term “machine” shall also be taken to include any collection of machines that individually or jointly execute a set (or multiple sets) of instructions to perform any one or more of the methodologies discussed herein.

The example computer system 700 includes a processing device 702, a main memory 704 (e.g., read-only memory (ROM), flash memory, dynamic random access memory (DRAM) such as synchronous DRAM (SDRAM) or Rambus DRAM (RDRAM), etc.), a static memory 706 (e.g., flash memory, static random access memory (SRAM), etc.), and a data storage system 718, which communicate with each other via a bus 730.

Processing device 702 represents one or more general-purpose processing devices such as a microprocessor, a central processing unit, or the like. More particularly, the processing device can be a complex instruction set computing (CISC) microprocessor, reduced instruction set computing (RISC) microprocessor, very long instruction word (VLIW) microprocessor, or a processor implementing other instruction sets, or processors implementing a combination of instruction sets. Processing device 702 can also be one or more special-purpose processing devices such as an application specific integrated circuit (ASIC), a field programmable gate array (FPGA), a digital signal processor (DSP), network processor, or the like. The processing device 702 is configured to execute instructions 726 for performing the operations and steps discussed herein. The computer system 700 can further include a network interface device 708 to communicate over the network 720.

The data storage system 718 can include a machine-readable storage medium 724 (also known as a computer-readable medium, such as a non-transitory computer-readable medium) on which is stored one or more sets of instructions 726 or software embodying any one or more of the methodologies or functions described herein. The instructions 726 can also reside, completely or at least partially, within the main memory 704 and/or within the processing device 702 during execution thereof by the computer system 700, the main memory 704 and the processing device 702 also constituting machine-readable storage media. The machine-readable storage medium 724, data storage system 718, and/or main memory 704 can correspond to the memory sub-system 110 of FIG. 1.

In one embodiment, the instructions 726 include instructions to implement functionality corresponding to search component 113 of FIG. 1). While the machine-readable storage medium 724 is shown in an example embodiment to be a single medium, the term “machine-readable storage medium” should be taken to include a single medium or multiple media that store the one or more sets of instructions. The term “machine-readable storage medium” shall also be taken to include any medium that is capable of storing or encoding a set of instructions for execution by the machine and that cause the machine to perform any one or more of the methodologies of the present disclosure. The term “machine-readable storage medium” shall accordingly be taken to include, but not be limited to, solid-state memories, optical media, and magnetic media.

Some portions of the preceding detailed descriptions have been presented in terms of algorithms and symbolic representations of operations on data bits within a computer memory. These algorithmic descriptions and representations are the ways used by those skilled in the data processing arts to most effectively convey the substance of their work to others skilled in the art. An algorithm is here, and generally, conceived to be a self-consistent sequence of operations leading to a desired result. The operations are those requiring physical manipulations of physical quantities. Usually, though not necessarily, these quantities take the form of electrical or magnetic signals capable of being stored, combined, compared, and otherwise manipulated. It has proven convenient at times, principally for reasons of common usage, to refer to these signals as bits, values, elements, symbols, characters, terms, numbers, or the like.

It should be borne in mind, however, that all of these and similar terms are to be associated with the appropriate physical quantities and are merely convenient labels applied to these quantities. The present disclosure can refer to the action and processes of a computer system, or similar electronic computing device, that manipulates and transforms data represented as physical (electronic) quantities within the computer system's registers and memories into other data similarly represented as physical quantities within the computer system memories or registers or other such information storage systems.

The present disclosure also relates to an apparatus for performing the operations herein. This apparatus can be specially constructed for the intended purposes, or it can include a general purpose computer selectively activated or reconfigured by a computer program stored in the computer. Such a computer program can be stored in a computer readable storage medium, such as, but not limited to, any type of disk including floppy disks, optical disks, CD-ROMs, and magnetic-optical disks, read-only memories (ROMs), random access memories (RAMs), EPROMs, EEPROMs, magnetic or optical cards, or any type of media suitable for storing electronic instructions, each coupled to a computer system bus.

The algorithms and displays presented herein are not inherently related to any particular computer or other apparatus. Various general purpose systems can be used with programs in accordance with the teachings herein, or it can prove convenient to construct a more specialized apparatus to perform the method. The structure for a variety of these systems will appear as set forth in the description below. In addition, the present disclosure is not described with reference to any particular programming language. It will be appreciated that a variety of programming languages can be used to implement the teachings of the disclosure as described herein.

The present disclosure can be provided as a computer program product, or software, that can include a machine-readable medium having stored thereon instructions, which can be used to program a computer system (or other electronic devices) to perform a process according to the present disclosure. A machine-readable medium includes any mechanism for storing information in a form readable by a machine (e.g., a computer). In some embodiments, a machine-readable (e.g., computer-readable) medium includes a machine (e.g., a computer) readable storage medium such as a read only memory (“ROM”), random access memory (“RAM”), magnetic disk storage media, optical storage media, flash memory components, etc.

In the foregoing specification, embodiments of the disclosure have been described with reference to specific example embodiments thereof. It will be evident that various modifications can be made thereto without departing from the broader spirit and scope of embodiments of the disclosure as set forth in the following claims. The specification and drawings are, accordingly, to be regarded in an illustrative sense rather than a restrictive sense. 

What is claimed is:
 1. A system comprising: a memory device comprising a content addressable memory (CAM) block comprising a plurality of key tables each storing a respective plurality of stored search keys; and a processing device, operatively coupled with the memory device, to perform operations comprising: receiving, from a requestor, an input search key and an indication of one of the plurality of key tables; identifying a match between the input search key and one of the plurality of stored search keys in the one of the plurality of key tables, the one of the plurality of stored search keys having an associated offset value indicating a location in a sorted string table (SSTable) corresponding to the one of the plurality of key tables; reading the offset value from the one of the plurality of key tables; and returning, to the requestor, the offset value read from the one of the plurality of key tables, wherein the requestor is to retrieve, from the location in the SSTable, data representing a value associated with the input search key.
 2. The system of claim 1, wherein the CAM block comprises an array of memory cells organized into a plurality of strings, each string storing one of the plurality of stored search keys and comprising a plurality of memory cells connected in series between a precharged match line and a page buffer, and wherein each of the plurality of memory cells is connected to one of a plurality of search lines.
 3. The system of claim 2, wherein the input search key comprises a first sequence of bits, and wherein the processing device is to perform operations further comprising: generating a search pattern based on the first sequence of bits, the search pattern comprising a first set of voltage signals representing the first sequence of bits and a second set of voltage signals representing a second sequence of bits comprising an inverse of the first sequence of bits.
 4. The system of claim 3, wherein the processing device is to perform operations further comprising: providing the search pattern as an input to the plurality of search lines of the CAM block, wherein the search pattern to cause at least one string of the plurality of strings storing the one of the plurality of stored search keys that matches the input search key to be conductive and provide a signal to the page buffer in response to the input search key matching the one of the plurality of stored search keys stored on the at least one string, the signal resulting from the precharged match line discharging, and the page buffer storing data based on the signal.
 5. The system of claim 1, wherein the memory device comprises a negative and (NAND) type flash memory device.
 6. The system of claim 1, wherein the processing device is to perform operations further comprising: receiving, from the requestor, the input search key and an indication of two or more of the plurality of key tables; and performing a search of the two or more of the plurality of key tables in parallel to identify the match between the input search key and the one of the plurality of stored search keys.
 7. The system of claim 1, wherein each of the plurality of key tables corresponds to a respective one of a plurality of SSTables.
 8. The system of claim 7, wherein the plurality of SSTables is maintained on a separate memory device.
 9. The system of claim 1, wherein the processing device is to perform operations further comprising: receiving, from the requestor, the input search key and an indication of a specific sub-block of the one of the plurality of key tables; and performing a search of the specific sub-block to identify the match between the input search key and the one of the plurality of stored search keys.
 10. A method comprising: receiving, from a requestor, an input search key and an indication of one of a plurality of key tables maintained in a content addressable memory (CAM) block of a memory device, each of the plurality of key tables storing a respective plurality of stored search keys; identifying a match between the input search key and one of the plurality of stored search keys in the one of the plurality of key tables, the one of the plurality of stored search keys having an associated offset value indicating a location in a sorted string table (SSTable) corresponding to the one of the plurality of key tables; reading the offset value from the one of the plurality of key tables; and returning, to the requestor, the offset value read from the one of the plurality of key tables, wherein the requestor is to retrieve, from the location in the SSTable, data representing a value associated with the input search key.
 11. The method of claim 10, wherein the memory device comprises a negative and (NAND) type flash memory device.
 12. The method of claim 10, further comprising: receiving, from the requestor, the input search key and an indication of two or more of the plurality of key tables; and performing a search of the two or more of the plurality of key tables in parallel to identify the match between the input search key and the one of the plurality of stored search keys.
 13. The method of claim 10, wherein each of the plurality of key tables corresponds to a respective one of a plurality of SSTables.
 14. The method of claim 13, wherein the plurality of SSTables is maintained on a separate memory device.
 15. The method of claim 10, further comprising: receiving, from the requestor, the input search key and an indication of a specific sub-block of the one of the plurality of key tables; and performing a search of the specific sub-block to identify the match between the input search key and the one of the plurality of stored search keys.
 16. A non-transitory machine readable storage medium storing instructions that, when executed by a processing device, cause the processing device to perform operations comprising: maintaining, a plurality of key tables in a content addressable memory (CAM) block of a memory device, each of the plurality of key tables storing a respective plurality of stored search keys from a corresponding plurality of sorted string tables (SSTables); responsive to a query comprising an input search key and an indication of one of the plurality of key tables, performing a search of the respective plurality of stored search keys in the one of the plurality of key tables for an entry matching the input search key, wherein the entry comprises an associated offset value indicating a location in a corresponding one of the plurality of SSTables; and providing, to a host system, the offset value read from the one of the plurality of key tables, wherein the requestor is to retrieve, from the location in the corresponding one of the plurality of SSTables, data representing a value associated with the input search key.
 17. The non-transitory machine readable storage medium of claim 16, wherein the query comprises a key prefix, wherein the entry in the one of the plurality of key tables comprises a first entry storing a stored search key matching the key prefix, and wherein the requestor is to retrieve a plurality of data values from the one of the plurality of SSTables starting at the location and continuing until keys in the one of the plurality of SSTables stop matching the key prefix.
 18. The non-transitory machine readable storage medium of claim 16, wherein the processing device is to perform operations further comprising: receiving, from the host system, the input search key and an indication of two or more of the plurality of key tables; and performing a search of the two or more of the plurality of key tables in parallel to identify the entry matching the input search key.
 19. The non-transitory machine readable storage medium of claim 16, wherein the plurality of SSTables is maintained on a separate memory device.
 20. The non-transitory machine readable storage medium of claim 16, wherein the processing device is to perform operations further comprising: receiving, from the requestor, the input search key and an indication of a specific sub-block of the one of the plurality of key tables; and performing a search of the specific sub-block to identify the entry matching the input search key. 